Micro-machined sensor and actuator devices having tuning fork resonators are generally well-known. See, for example, FIG. 1 which illustrates a sensor device 1 of the type taught by U.S. Pat. No. 6,145,380, SILICON MICRO-MACHINED ACCELEROMETER USING INTEGRATED ELECTRICAL AND MECHANICAL PACKAGING, which is incorporated herein by reference and is further co-owned by the assignee of the present application. U.S. Pat. No. 6,145,380 teaches an integrated sensor package having an acceleration sensor 1 formed for example as a double-ended tuning fork (DETF) resonator-based acceleration sensor. A typical DETF resonator-based acceleration sensing mechanism includes a frame 10 formed of a suitable substrate material, for example, silicon or quartz. A reaction mass or proof mass 12 is rotatably suspended from frame 10 by one or more hinges 14 for rotation about hinge axis 16. One or more force sensing DETF resonator 18 is suspended between frame 10 and reaction mass 12. As reaction mass 12 rotates relative to frame 10 in response to a force experienced along input axis 20, DETF resonator 18 experiences either a compressive or tensile force along its longitudinal axis 22. In other words, DETF resonator 18 is either compressed or stretched between frame 10 and reaction mass 12 when reaction mass 12 is displaced or rotated away from a null position relative to frame 10. The natural frequency of DETF resonator 18 changes when it is compressed or stretched: the natural frequency decreases below a nominal resonance frequency when DETF resonator 18 is compressed, and increases above a nominal resonance frequency when DETF resonator 18 is stretched. The resulting change in frequency is proportional to the force or acceleration applied to reaction mass 12. This push/pull phenomenon is extensively described in U.S. Pat. No. 5,005,413, which is incorporated herein by reference.
FIG. 2 is a detailed example of DETF resonator 18 formed as a two-tine vibrating beam force sensing DETF resonator. Micro-machined silicon acceleration sensor 1 may employ vibrating beam force sensing resonators of, for example, the general configuration shown in FIG. 4. DETF resonator 18 is formed of two tines 24, 26 attached to mounting tabs 28, 30.
In a typical state of the art micro-machined sensor and actuator devices such as acceleration sensor 1, DETF resonator 18 is typically suspended by mounting tabs 28, 30 between frame 10 and reaction mass 12 across hinge 14. Tines 24, 26 are adapted to vibrate or oscillate at their respective natural frequencies in response to an electronic drive signal applied by a drive circuit.
FIG. 3 illustrates the mechanical operation of DETF resonator 18. U.S. Pat. No. 6,145,380 describes several known methods for inducing oscillation in tines 24, 26. For example, tines 24, 26 may be adapted to accept an electrical current by forming them in a semiconducting material, such as doped conductive polysilicon. In another example, electrically conductive film electrodes may be deposited on a surface of tines 24, 26. Vibration or oscillation of tines 24, 26 may be accomplished by various means. For example, in a typical magnetic drive sensor, tines 24, 26 are mounted within the field, B, of one or more permanent magnets (not shown). A drive circuit applies an oscillating or alternating current, I, in electrically conductive film electrode 32, which induces a sympathetic alternating magnetic field within the conductive film electrode. The alternating or oscillating current-induced magnetic field in the conductive film electrode interacts with the field of the permanent magnets to create forces, F1 and F2, which drive tines 24, 26 into oscillation.
In an alternate configuration (not shown), DETF resonator 18 may be manufactured having four tines. In a four tine resonator, the sensing circuit may have two pair of driven and sensed tines, each pair having an inner tine and an outer tine as described in U.S. Pat. Nos. 5,367,217 and 5,331,242, both incorporated herein by reference.
Alternatively, in an electrostatic or capacitive drive vibratory system 34, as illustrated in FIG. 4. As described in U.S. Pat. No. 6,145,380, tine oscillation may be driven by inducing alternating or oscillating electrostatic forces between electrically conductive surfaces on tines 36, 38 and adjacent conductors 40, 42 mounted on frame 10 adjacent to and coextensive with conductive surfaces on tines 36, 38. Force transducers based upon electrostatically driven DETF resonators are known in the art and are disclosed by example and without limitation in U.S. Pat. Nos. 4,901,586; 5,456,111 and 6,745,627, all commonly assigned to the assignee of the present application and all incorporated herein by reference. Other examples of micro-machined silicon acceleration sensors which may be used with the present invention are described in U.S. Pat. Nos. 4,766,768 and 5,241,861, both commonly assigned to the assignee of the present application and both incorporated herein by reference.
However, in configurations having DETF resonator 18 suspended between frame 10 and reaction mass 12 across hinge 14 with both ends of the DETF resonator 18 being driven, damping loads developed in the physical connection may generate errors in resonator output.
Therefore, devices and methods for overcoming these and other limitations of typical state of the art micro-machined sensor and actuator devices, such as typical state of the art MEMS accelerometer devices, are desirable.